1. The Field of the Invention
This invention relates to devices that detect an open ground connection, more particularly to a device which automatically alerts operating personnel upon detection of an open ground connection occurring anywhere between the chassis of an electrical appliance and the grounding terminal of a standard wall receptacle.
2. The Prior Art
The use of modern electrical appliances presents a potential shock hazard to any person using such an appliance. Fortunately, the design and manufacture of modern electrical appliances minimizes this hazard. Still, wear, abuse or damage to an appliance may result in a voltage potential being placed on a conducting surface accessible to a user. With the common supply line voltages between 110 and 125 volts rms, and current carrying capabilities of 15 amperes and greater, any shock that may occur is potentially a lethal one. Furthermore, static charge accumulations and stray currents created by capacitive and inductive coupling present a shock hazard under some circumstances.
In order to avoid these shock hazards, both industry and government agencies have promulgated regulations requiring that many appliances be provided with an independent grounding connection in cooperation with two or more current supplying conductors. In a conventional electrical distribution system, the most prevalent means of supplying a grounding connection is the use of an independent conductor, referred to as a grounding conductor, which is used to connect the appliance chassis to the grounding bus of the permanent wiring system. The grounding conductor is in addition to, and independent of, the hot conductor (or conductors) and the neutral conductor (if used) which apply voltage and carry current to the appliance.
The conductors used to connect an appliance to the permanent wiring system are typically arranged within a single covering generally referred to as the power cable. Two of the most common methods of providing the necessary detachable connections between power cable conductors and their respective hot, neutral and ground buses of the permanent wiring system are known in the art as a nonlocking "straight blade" plug in conjunction with a corresponding "straight blade" receptacle or a locking "twist-lock" plug in conjunction with a "twist-lock" receptacle. In the case of a two wire system, where a hot, neutral and grounding bus are provided in the permanent wiring system (i.e., three wire connection) three blades are provided on the plug. By far the most common device used to make detachable connections with a three wire permanent wiring system is known in the art as a 15 amp, 125 volt, 3 wire, grounding, straight blade plug and its corresponding receptacle. In other systems using more than one hot and one neutral bus, such as three and four wire systems, whether single or polyphase, the plug and receptacle is configured so as to provide an independent grounding connection.
With a grounding line properly connected to a grounding bus and to the appliance, if a line voltage is placed on an exposed conducting surface or chassis of an appliance, the properly connected grounding conductor will provide a very low resistance path to ground. Because of the low resistance path to ground, current flow less than the rating of the circuit breaker and fuse will minimize potential chassis voltages with respect to ground. Current flow exceeding the designed rating of the line will trip the circuit breaker or fuse, thus interrupting the voltage and current to the appliance. Considering the severity of the potential shock a person may receive when contacting a 125 volt rms potential, the importance of maintaining the integrity of a proper ground is readily apparent.
Either through abuse, damage or normal wear, a proper ground connection may become inoperative. The most common cause of failure of the ground connection lies with a lack of continuity of the grounding conductor of the power cable or the lack of continuity between the plug grounding blade and the receptacle grounding terminal. The danger presented by an open ground connection is high because it is generally unnoticeable by a casual observer, and unlike the loss of a neutral or hot conductor, most appliances will function normally without a proper ground connection to the chassis of the appliance. Therefore, not only does a potential shock hazard exist, but it is undisclosed until a shock occurs or until the ground connection is examined. Such a condition poses a serious threat in any circumstance, but such a condition is particularly dangerous in a health care facility such as a hospital.
Under most circumstances a current of greater than 50 milliamperes passing through the body trunk of a healthy person is necessary to cause severe pain or injury. However, some authorities claim that currents as small as 10 microamperes, when passed directly through the heart, may cause ventricular fibrillation.
In a hospital setting the patients, many of whom are immobile and/or in a weakened physical condition, are particularly susceptible to the hazards of an electric shock. Since such patients are often intimately connected to life support systems which in turn require connection to electrical systems, this problem requires extreme care and constant surveillance.
Shock hazards may be caused by problems in an electrical distribution system, improper wiring, by an appliance malfunction, by leakage currents created by capacitive and inductive coupling between current carrying conductors and various structures, or even by static electricity. Thus, proper grounding not only guards against the potential shock hazard from equipment leakage currents or electrical system malfunction, but also protects static sensitive semiconductor devices and electrical equipment from destruction due to electrical static discharge.
As noted above, while the dangers of electric shock are not unique to a hospital environment, there is an acute need in that environment to insure that the protection afforded by a properly grounded system is present. Because of the importance of insuring that there is a proper ground connection, several safety measures have been devised.
One approach, and perhaps the most common, is for a qualified technician to periodically check the continuity of the ground connection between the chassis of the appliance and the grounding blade of the plug. Several safety organizations require that such periodic tests be made. For example, the National Fire Protection Association suggests in the case of portable patient care equipment that the resistance from appliance chassis to plug grounding blade be checked twice each year and that it be below 0.15 ohms. NAPA 76B, 1980 ed., para. 4-4.5.2. The Joint Committee on the Accreditation of Hospitals ("JCAH") also requires that equipment be performance tested twice each year, including the grounding system. 1982 JCAH Accreditation Manual for Hospitals, page 37.
While the periodic test approach is commonly used, it has several serious drawbacks. First, there is nearly always a significant period of time between the occurrence of a malfunction and the next periodic inspection. Second, manual inspections by a technician may not detect malfunctions that occur only intermittently. Third, in such checks, the integrity of the connection between the plug grounding blade and the receptacle grounding terminal is not checked. It is often the case that a grounding blade and receptacle grounding terminal makes a marginal physical, and thus electrical, connection because of wear or damage. Fourth, in addition to the above mentioned drawbacks, use of technicians to make periodic checks and maintain records requires technician time and is an additional operating expense.
Another approach in the prior art is to provide a redundant connection between the appliance chassis and the grounding bus and then monitor the continuity of the connection. This approach requires that an appliance be provided with a primary grounding connection integral with the power cable plug and receptacle combination. A redundant connection is then provided from the appliance chassis to the permanent wiring system grounding bus at a point away from primary grounding connection to the bus. A small alternating current, generally above 10 KHz and below 100 KHz, is induced in the closed circuit created by the primary grounding conductor, the appliance chassis, the redundant grounding conductor and the grounding bus. The current flow in the redundant grounding conductor is monitored. An increase in resistance in either of the grounding conductors is indicated by a drop in the current flow.
A drawback of this approach is that it requires the use of a separate, redundant ground which increases cost and generally requires a separate grounding conductor which adds to the number of cords "running" in an area from the appliance to the permanent wiring system ground bus. See 1981 NAPA 70 Article 517 para. (c) (1-4) (deleting the standard of 1974 NAPA 56A requiring "equi-potential grounding systems" in which a redundant ground was often required).
Still another approach has been to place a DC or 60 Hz line current on a single grounding line and provide a means for indicating and/or disconnecting the neutral and hot conductors when the current drops below a predetermined threshold indicating an increase in the ground resistance.
The major disadvantage of this method is that the devices generally use relays, or semiconductor devices that require an unsatisfactorily high current flow on the ground conductor for proper operation. Furthermore, such devices do not provide a means to indicate impending plug grounding blade and receptacle grounding terminal failure. Thus, such devices do not provide adequate assurance against shock hazards, especially in the environment of a hospital.
The present invention overcomes the disadvantages of the prior art and introduces several additional benefits to the art.